Recently, a short-range radar using UWB has nearly come to find practical application for use on an automotive vehicle and for visually handicapped persons.
In the short-range radar of this type using UWB, like in the ordinary radar, the short pulse wave is emitted from the antenna of a transmitting unit into the space, and a receiving unit receives the wave reflected from an object existing in the space thereby to analyze the object.
FIG. 24 is a block diagram showing the general configuration of the transmitting unit of the conventional short-range radar of this type.
Specifically, in the transmitting unit of this short-range radar, a carrier signal S of a predetermined UWB frequency output continuously from a carrier signal generator 1 is input to a switch circuit 2.
The switch circuit 2 is intermittently turned on/off by a pulse signal Pa output from a pulse generator 3 in a predetermined period thereby to generate a short pulse wave (burst carrier) Pb.
This short pulse wave (burst carrier) Pb, after being amplified by an amplifier 4, is emitted into the space from an antenna 5.
The aforementioned configuration in which the short pulse wave Pb is generated by intermittently turning on/off the switch circuit 2 inserted into the path of the carrier signal S, however, poses the problem that the carrier signal output, though ideally completely stopped during the turn-off state of the switch circuit 2, cannot be actually stopped completely due to the leak of the switch circuit 2.
The turn-off period (for example, 1 μs) of the switch circuit 2 is normally very long as compared with the turn-on period (for example, 1 ns) thereof. On the average, therefore, the power of the carrier leak is not ignorable.
Especially, it is difficult to prevent the leak of the carrier signal in the UWB of high frequency.
Therefore, the spectrum density Sx of the short pulse wave Pb, as shown in FIG. 25, for example, is such that the leak component S′ is projected considerably at the position of the carrier frequency fc.
This leak component S′ limits the real receiving sensitivity of the reflected wave of the short pulse wave Pb output at the regular transmission timing. As a result, the radar search range is narrowed, and it becomes difficult to detect an obstacle of a low reflectivity as an object existing in the space.
With regard to the UWB radar system described above, FCC (Federal Communication Commission of USA) provides, in Non-Patent Document 1 described below, the spectrum mask as shown in FIG. 26.
This spectrum mask, revised and published on Dec. 16, 2004, constitutes a standard much stricter than the first one disclosed on Feb. 14, 2002 in Non-Patent Document 2 described below.
In this revised spectrum mask, the power density of UWB in the range of 22.0 to 23.12 GHz and the range not lower than 29.0 GHz is specified at −61.3 dBm/MHz or less on the one hand, and the power density in the range of 23.12 to 23.6 GHz and the range of 24.0 to 29.0 GHz is specified at −41.3 dBm/MHz or less.
Also, in the frequency range of 23.6 to 24.0 GHz or what is called the radiowave emission prohibited band (RR prohibited band) or the radiowave emission restricted band (RR restricted band) where the radiowave emission is intentionally prohibited under the International Radio Communication Regulations (RR) to protect the passive sensor of the radio astronomy or the Earth Exploration Satellite Service (EESS), the emission power density is suppressed to −61.3 dBm/MHz lower by 20 dB than in the past.
In the spectrum mask described above, the total energy amount in each predetermined band is restricted to not more than a specified value. In the case where the leak component S′ is large as in the aforementioned case, therefore, the output of the short pulse wave Pb for the regular transmission timing is required to be set at a correspondingly low level, resulting in a considerable limitation of the search range of the radar.
In view of this, a solution to the problem of the leak component S′ has been conceived, in which as shown in FIG. 26, the carrier frequency of the short pulse wave Pb is made to coincide with the UWB frequency band of 24.05 to 24.25 GHz for the Doppler radar (Short Range Device: SRD) where the emission of the power of a higher level than −41.3 dBm/MHz is permitted.
In the neighborhood of this SRD band, however, the aforementioned RR prohibited band exists. Further, as described above, the short pulse wave Pb constituting the pulse modulation signal as a carrier signal turned on/off intermittently by the pulse signal has a spectrum width of several hundred MHz to 2 GHz.
In the case where the carrier frequency is set at the SRD band in the neighborhood of the RR prohibited band as described above, therefore, the considerably high level portion of the spectrum of the short pulse wave Pb is superposed with the RR prohibited band. Actually, therefore, it is very difficult to suppress the emission power density to −61.3 dBm or less specified by the latest spectrum mask.
Also, the first FCC standard permitting the emission level in the RR prohibited band up to −41.3 dBm/MHz stipulates that in order not to interfere with the EESS described above, the emission strength of the radiowave used for other purposes at the angle of emission direction (direction of elevation) larger than 30° from the normal to the globe surface is required to be lower than the emission strength in the emission range of 0° to 30° by −25 dB or more (in and after January, 2005). This standard has since become increasingly stricter for each several years.
In the case where the carrier frequency is set in the SRD band as described above, therefore, the side lobe of the vertical surface of the antenna is required to be suppressed so as not to increase the direction of emission of transmitted radiowave.
The suppression of the side lobe on the vertical surface of the antenna, however, requires the arrangement of a number of antenna elements along the height into an array. This increases the size along the height and makes an application as an on-vehicle radar difficult.
Also, in order to avoid the problem of the leak component S′, various methods have been conceived to improve the isolation of the switch circuit 2.
Even in the case where a switch capable of achieving a high isolation in the very high frequency band described above can be realized, however, such a switch is very expensive and very difficult to employ for the on-vehicle radar used by common people or visually handicapped persons.
Non-Patent Document 1: FCC 04-285 “SECOND REPORT AND ORDER AND SECOND MEMORANDUM OPINION AND ORDER”
Non-Patent Document 2: FCC 02-48, “FIRST REPORT AND ORDER”